Rotary mass-imbalance systems can be defined as systems in which a shaft or other member is rotatable about an axis by a motor or the like and is constructed with, or carries a mass whose center of gravity is offset from the axis of rotation. Since the unbalanced mass acts at its center of gravity or center of mass and the latter does not coincide with the axis of rotation of the shaft or body the combined center of gravity of shaft and the mass is eccentric and an oscillation is imparted to the structure in which the shaft is journaled.
Such mass-imbalance rotary system serve as oscillatory drives for feed machines (bulk-solid metering devices), transport apparatus and material-working machines, such as deburrers, polishers, grinders and the like.
Generally speaking one attempts with rotary systems to eliminate all imbalance or to minimize imbalance so as to insure uniform rotation, minimum vibration and minimum wear of the apparatus, in oscillatory drives of the type described above, (eccentricity) such imbalance is purposely created. Depending upon the throw of the umbalanced mass, the magnitude of the unbalanced mass and the speed of rotation, practically any desired oscillatory system can be created with rotary unbalanced masses.
The apparatus can include containers, troughs, tools or the like which are set in motion by the oscillatory drive for various purposes.
For example, devices have been proposed for the surface treatment of workpieces by particles of solid abrasive material, liquid suspension medium, chemically active liquids and polishing or grinding agents of various types, in which the articles to be treated and the treating agent or agents are introduced into a container which is subjected to an oscillatory movement. As a result of the relative movement between the components of the mixture it is possible to obtain polishing, grinding, smoothing and abrading or deburring of the workpiece.
For the most part, an apparatus of this type can comprise a frame, foundation or base upon which the container is mounted for oscillatory movement. An oscillatory drive of the character described is used to displace the container and auxiliary devices can be provided for delivering the components of the mixture to the receptacle and emptying the latter.
In most surface treatment systems, the oscillatory drive comprises an electric motor carrying a shaft which is journaled with minimum play relative to the motor and upon which an unbalanced mass is mounted. The shaft is journaled in bearings fixed to the container so that the oscillations of the shaft are transmitted by these bearings to the container and the latter undergoes an oscillatory movement. When the container is formed as an annular trough, the oscillatory drive has a vertical shaft disposed centrally of the trough.
For various reasons it has been desirable to establish different oscillatory-movement status, e.g. to alter the type, degree or rate of surface treatment, to accommodate articles of different sizes, to permit optimum results with different fluids etc. Frequently it is desirable to treat the articles successively or selectively at a low rate and at a high rate (corresponding to one and another oscillation condition respectively) or to vary the circulation or feed action in the container.
These requirements have been fulfilled heretofore in a system in which replaceable unbalanced weights of different dimensions were releasably mounted upon the rotaton system. Furthermore the removal and attachment of weights permits them to be mounted in different angular relationships to one another for various purposes.
Because the entire rotation system, in use, constantly is being vibrated, all of the parts thereof are subject to relatively large wear and must be of simple and strong construction to allow economical replacement or to enable them to withstand the stresses. This of course applies especially to those structural elements which have been removable and replaceable. Hence, when removable weights are used, the entire structure must be made stronger and more massive than would otherwise be required. Another disadvantage of the prior art system is that the adjustment of the degrees of imbalance, replacement of the weights and positioning thereof is a complicated manipulative proceeding and increases the difficulties of operating the apparatus. Not only does the adjustment require considerable time and skill, but, for safety reasons, the adjustment portions must be shielded or enclosed to prevent the operator from coming into contact with the moving parts. In practice, the apparatus must be equipped with lockout controls or the like to prevent the system from being set into motion by an operator while adjustment is in progress.
It has been proposed, in an effort to eliminate the last-mentioned difficulties, to provide an oscillatory drive with pneumatic, hydraulic or other auxiliary means for selectively positioning a weight between two different orientations or locations. However, even this arrangement has significant disadvantages. For one thing, the energy necessary to operate the auxiliary mechanism must be fed to an oscillating system, thereby raising the possibility of damage and interruption of the system or damage to the auxiliary devices. Apart from the significantly increased cost of such devices, there is the disadvantage that devices which rotate with the system and are supplied by an external source create technical difficulties with respect to design and construction.
To position an imbalance weight between two alternate locations in a rotation system for the purpose described, it has also been proposed to utilize the reversing effect of the drive electric motor so that the weight assumes one position when the motor is rotating in one sense and the alternative position when the motor is rotated in the opposite sense. This arrangement has the disadvantage that the bearings are stressed and subjected to more wear with motor reversal than with a system in which the motor operates only in a single sense.